Cob: Re: Cobber's Thumb and Knitting cob together

[Cobworks]

Hello cob list;
     The system I have been using on my cob projects to get a good bond from
one days cobbing to the next is as follows. I always leave the wall at the
end of the day rough and full of holes.  If drying has been a problem I will
spine and rib the wall.  In the morning  I take a stiff brush and thoroughly
wet the top of the wall.  With my first load of cob I take a handful and dip
it in water which makes a wet cob.  I then take this "adhesion" cob and work
it into all the holes and rough surface on the wall.  This is a very thin
coat that bonds well to the last layer.  I then begin Gaab cobbing on the
wall.  I get good bonding and the process of building the wall gets speeded
up.                                                       Patrick on Mayne

----- Original Message -----
From: Shannon C. Dealy <dealy at deatech.com>
To: <coblist at deatech.com>
Sent: Wednesday, July 26, 2000 11:43 AM
Subject: Cob: Cobber's Thumb and Knitting cob together


> On Wed, 26 Jul 2000 goshawk at gnat.net wrote:
>
> > it's a stick used to make holes in top of a level of cob so that the
next level will hold better.
> > I've not been using that technique on my walls. I generally wet the
level I'm adding to if it's dry and
> > sometimes use my fingers to punch a few holes but then more often  than
not haven't  and have
> > not had any problems with my technique at the this point.
>
> The point of using your thumb or a "Cobber's thumb" is to get a good
> interlock between the different layers of cob.  In a given lump of cob
> mix, the straw fibers are all twisted around each other, making it into a
> single integrated piece.  When you place a cob on top of a wall, it is not
> tied into the rest of the wall until you repeatedly push your thumb
> through the cob you are adding and into the wall below it.  By doing this,
> part of the fibers from the cob you are adding become intermixed with the
> cob below, binding the layers together.  If you don't do this, you end up
> with the layers being held together by friction and gravity, which is fine
> until the wall encounters some type of horizontal shear force such as a
> car hitting the building, earthquake or other disaster, at which point,
> any cob which has not been knitted together by this means provides
> built-in fracture points for the wall to break apart.  While some people
> disagree that this is a necessay proceedure, my approach is always to
> build for maximum structural strength in all directions, so I always knit
> the layers together in this manner.
>
> With regard to putting holes in the top of the cob, there are two reasons
> that I know of for doing this other than the incidental creation of holes
> in the cob while knitting the layers together (making holes is not the
> point of the knitting process).  First, putting holes in the cob will
> provide more exposed surface area which will help the cob to dry faster.
> In warm dry areas, this is not generally a problem, but if you are cobbing
> in 60 degree (F) or colder weather or at a humid or damp time of year, it
> may be necessary.  The second reason is that if you need to stop work on a
> wall for a period of time during which the top of the wall may dry and
> harden, in most cases you will not be able to get the wall to soften up
> enough for knitting the next layer to the wall when you start work again.
> By putting lots of holes in the top of your wall before you leave it, you
> provide a mechanical interlock that the next layer of cob can be forced
> into which will tie the wall together much better than if the new cob were
> simply added to a flat surface.  Doing this will NOT give you as good a
> bond as knitting together two layers of soft cob, but it will give you a
> good bond.  NOTE: before applying fresh cob to a dried wall, always soak
> the surface you are going to apply the cob to with lots of water, possibly
> over several days, in most cases, the dried cob won't get soft, but it
> should get a little softer, and in either case the new cob will bond to it
> much better.
>
> Now to answer one final obvious question, why use a cobber's thumb instead
> of your own?  Many people don't, but working with a good high sand cob mix
> and pushing your thumbs through it all day long does tend to wear the skin
> off a bit :-)  Some people will just grab any convenient stick of about
> 1" diameter for use as a cobber's thumb, and others will buy or make their
> own.  The primary advantages of a cobber's thumb that is made for that
> purpose over a convenient stick, is that sticks (at least the ones I find
> lying around) often are not particularly hard/strong and a day of cobbing
> can quickly grind some woods down to nothing, so a manufactured cobber's
> thumb made from a good strong seasoned wood will last longer, and it can
> be made to comfortably fit the palm of your hand which may be a little
> easier on the hands over time (I find that sticks rarely come with
> convenient palm shapes :-)  I personally tend to think that people should
> get their hands into the mix at least some of the time to get a real feel
> for what they are doing, but when I work with a good high sand cob mix
> for any period of time, I will eventually grab a cobber's thumb or stick
> in order to leave some skin on my fingers at the end of the day :-)
>
>
> Shannon C. Dealy      |               DeaTech Research Inc.
> dealy at deatech.com     |          - Custom Software Development -
>                       |    Embedded Systems, Real-time, Device Drivers
> Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications
>    or: (541) 451-5177 |                  www.deatech.com
>
>
>